This invention relates to a magnetron having an internal capacitor for suppressing high frequency leakage and, more particularly, to improvements in the cathode stem structure of a magnetron.
A known magnetron produces a fundamental high frequency wave, for instance 2,450 MHz. In the magnetron high frequency noises in a wide frequency range from several KHz to GHz is generated from various grounds. The fundamental high frequency wave and the high frequency noises at harmonic frequencies and other high frequencies in the UHF band, VHF band and radio band are likely to leak from the magnetron, particularly from a cathode terminal of the magnetron cathode stem, and adversely affect other electronic equipment and human body. In order to prevent the fundamental frequency and high frequency noise leakage, the cathode stem of the magnetron is received in a shield box, and the cathode terminals of the cathode stem are electrically connected to LC filters. The impedance of these LC filters is generally determined in dependence upon the high frequency noise to be suppressed. However, even if the LC filters are properly selected to have a proper value against noises, the LC filters affect the oscillation of the magnetron and also cause a standing wave to be produced in power supply lines connected to the cathode terminals, if the power supply line including LC filters is improperly designed.
If, for example, the power supply line is undesirably designed, an unrequired resonance path including power supply line is formed at the fundamental high frequency and an output from an output section becomes unstable. At the time of presence of resonance the microwave output from the output section of the magnetron is likely to reduce the efficiency of the magnetron. Further, at the time of the resonance electrons are likely to intensively bombard the cathode in the resonance cavity to deteriorate emission of electrons from the cathode. Furthermore, depending upon the impedance of the LC filter it is likely that the high frequency noise spectrum is changed to shift the maximum level noise frequency range from the intended range to another particular range, thus practically disabling the suppression of high frequency noise with the LC filters. Further, even if the impedance of the LC filters is adequately selected with respect to the high frequency noise, considerable amount of microwave power is likely to be absorbed by the LC filters thereby burning the choking element of the LC filters. Because the method of suppressing high frequency noise by connecting LC filters to the cathode terminals of the cathode stem has various problems as mentioned above, there has been proposed a magnetron having internal capacitors formed within the cathode stem, disclosed in U.S. Pat. No. 4,163,175, applied by Tashiro, the inventor of the invention of the instant application. In this magnetron, a first internal capacitor is formed between cathode holders within the cathode stem for the purpose of attenuating high frequency noise transmitted through heater lines through which heater current is supplied to the cathode. In addition, a second internal capacitor is formed between a cathode holder and an anode cylinder within the cathode stem in order to attenuate high frequency noise transmitted through high voltage supply lines through which a high voltage is supplied between the cathode and anode. With this magnetron provided with the first and second internal capacitors, disclosed in U.S. Pat. No. 4,163,175, particularly the fundamental high frequency produced in the resonance cavity of the magnetron can be suppressed by the first and second internal capacitors, so that it is possible to comparatively freely select the impedance of the LC filters connected to the cathode terminals of the cathode stem and thus effectively suppress the high frequency noise. However, the disclosed magnetron has a problem in that a high voltage which is applied between the cathode holder and anode cylinder, is likely to cause dielectric breakdown of the second capacitor and thus cause surface discharge or creeping discharge through the surfaces of the dielectric member constituting the second capacitor.